Increasing levels of integration of integrated circuit chips reduces the chip count of a functional circuit, while significantly increasing the I/O count of the individual integrated circuits making up the functional circuit. This drive for increased circuit and component density in the individual integrated circuit chips leads to a parallel drive for increased circuit and component density in the printed circuit boards carrying the chips and in the assemblies using them.
The increased circuit and component density in the printed circuit boards makes the ability to locate either solder surface mount components or place additional circuitry layers directly above plated through-holes highly desirable. This is especially the case when the density of the plated through-holes required to service the I/Os of the surface mount components is such that there is no surface area available for attachment pads interstitial to the plated through-hole grid.
This problem is especially severe with fine pitch ball grid array components (BGA) and flip chip attach (FCA) integrated circuits. Soldering of the surface mount components to the surface pads, i.e., LANs of conventional plated through-holes is highly undesirable. This is because the solder used for assembly tends to wick down into the plated through-holes. The result is low volume, unreliable solder joints.
One solution to this problem that has been proposed is filling the plated through-holes with a solid material, and if the fill material is not electrically conductive or not solderable, then to coat the surface of the filled hole with a conductive, solderable material such as by overplating. Compositions that have been suggested as the plated through-hole fill material have been selected based on certain criteria. In particular, it is desirable that the material have chemical and thermal compatibility with the printed circuit board fabrication processes and materials and particularly compatibility with the various plating baths employed. Also, the material should exhibit sufficient flow characteristics in order to fill small, high aspect ratio plated through-holes and have the ability to be transformed or converted into a solid material, with a minimal volume change after filling. The thermal expansion of the composition should be compatible with the rest of the printed circuit board structure. Furthermore, the composition should exhibit good adhesion to the metallization within the barrel of the plated through-holes.
One particular class of materials which can be formulated to meet the above criteria is the class of thermosetting resins filled with inorganic powders, and especially epoxy resin compositions of the same type used to fabricate the epoxy/glass prepregs used for printed circuit board construction, filled with metal powders such as copper.
The plated through holes could be filled by dispensing the fill material from a syringe, through a needle, into the holes. However, this procedure is rather time consuming and expensive and therefore not especially desirable from a commercial viewpoint. A method for mass filling of plated through-holes such as using a squeegee to force the fill material into the plated through-holes is difficult to carry out and to achieve complete fill of especially high aspect ratio holes. Furthermore, there exists a significant number of missed holes when filling with a stencil or screening and appreciable amount of surface or internal voids occurs.
A further problem also occurs, if it is desired to have both filled and unfilled plated through-holes on the same printed circuit board. It has been suggested to fill only selected plated through-holes by drilling holes in the mask sheet only in positions corresponding to the plated through-holes to be filled, leaving the plated through-holes to be unfilled blocked during the fill operation. The problem experienced however is that the fill material, which bleeds under the mask, can also bleed into plated through-holes which are to remain clear. This would not be a major concern if component leads or pins were not required to be inserted into the plated through-holes. However, the reason for requiring plated through-holes to be unfilled is to permit such assertion, and soldering of the pin/leads is also usually required. The film material which bleeds into pin-in-hole (PIH) plated through-holes will interfere with solder wetting in the barrel of the plated through-hole and could possibly constrict the diameter of the holes sufficiently to prevent the lead insertion.
A suggested process of selective drilling and filling involves the drilling and plating only of the plated through-holes to be filled before applying the hole fill process. The unfilled plated through-holes are drilled after filling, either before or after the nub and bleed remove operation. However, the processes used for overplating the plugs and the filled plated through-holes will also plate the barrels of the newly drilled, unfilled holes.
It would also be desirable to provide a process for selective plated through-hole filling whereby the solderability and cleanliness of the unfilled plated through-hole barrels is not compromised.